An update:

A friend passed me in his car on the way into work and said that the dynamo wasn't out completely; he saw a dim orange glow from the headlight, which makes me think it's not a blown bulb, but that the light isn't getting enough juice... any ideas behind why this would happen?

A few notes: we got freezing rain in the last 24 hours and there was a hard candy coating of ice over the entie bike, including the headlight.